When it is desired to make a diaphragm wall, i.e. a wall made of concrete in a trench dug in the ground, this operation is performed in successive segments, each corresponding to a panel of the diaphragm wall, and the set of panels constitutes the wall.
To provide mechanical continuity between wall panels in a zone where two panels meet, the end of each panel is given a special "joint" shape enabling the end of each panel to be received in the end of the preceding panel. This shape corresponding to the joints is defined at the end of the trench that is being dug by inserting formwork of a shape that forms the joint of the panel that is being made.
With reference initially to FIGS. 1A and 1B, there follows a description of the prior art technique for making the various panels of a diaphragm wall. Initially, a first portion of trench 10 is dug corresponding to a first panel, and end joint formwork 12 is placed at the end 10a of the portion of trench. In conventional manner, the formwork 12 comprises a soleplate 14 and a box 16 defining the shape of the joint, and preferably also comprises a structure 18 enabling a connection element 20 to be held and protected between sealing elements that can be provided in the diaphragm wall.
As shown in FIG. 1A, the concrete constituting the panel 22 passes round the ends of the soleplate 14 of the formwork 12 in side zones referenced 24. Some of this concrete can even end up against the outside face 14a of the soleplate 14, as shown at 26.
In the following step, as shown in FIG. 1B, a second portion of trench 28 is dug to constitute the following panel of the diaphragm wall. During this digging, the bucket of the mechanical digger can be used to scrape the outside wall 14a of the soleplate 14 so as to remove the corresponding portion of concrete. However, experience shows that there always remains some of the concrete that has passed round, as referenced 24.
Prior to filling the second portion of trench 28, it is of course necessary to extract the formwork 12. This operation is difficult because of the adhesion between the concrete and both with the box 18 and with the inside face 14b of the soleplate 14. In addition, this operation is made even more difficult because of the presence of concrete that has passed round in the zones 24.
To solve that problem, proposals have already been made in French patent 2 613 395 to coat the face of the formwork that faces into the concrete in a thin material that can be abandoned in the trench so as to avoid adhesion between the concrete and the formwork, and to extract the formwork substantially vertically.
Proposals have also been made to solve the same problem in French patent 2 647 828 by applying a shock to the joint on its side remote from the concrete by means of a tool that is guided by the joint itself.
Another known technique for removing the formwork 12 consists in using removal "hooks" which consist, as explained in greater detail below, essentially in two hook-shaped pieces which are engaged on the edges of the soleplate of the formwork and whose active portions are engaged against the inside face of the soleplate at its edges. FIG. 1C shows the general shape of such removal hooks 30 and 32 when in place.
FIG. 1D shows a removal hook 30. It comprises one end 31 for connection to a machine, an arm 33 for passing round the side edge of the soleplate 14, and a flange 30a for insertion between the inside face 14b of the soleplate and the concrete in order to unstick the formwork from the concrete.
Because of the presence of concrete in the zones 24 where it passes round the edges of the soleplate, it is very difficult to engage the removal hooks 30 and 32. The hooks need to clear themselves a passage round the edge zones 24 over a width of 2 cm to 3 cm (referenced e) corresponding to the width of the removal hooks, and to do so over a length that may exceed 5 cm (referenced e') corresponding to how far the concrete has gone past in the zones 24. This operation puts very large stresses on the removal hooks and can lead to them breaking or at least to wearing out very prematurely. In addition, jamming phenomena make this operation difficult and thus lengthy, thereby giving rise to non-negligible cost in the construction of the diaphragm wall.